An optical frequency comb is light with an optical spectrum consisting of multiple discrete frequency components. The frequency difference between adjacent frequency components is substantially constant and called the frequency offset. The phase difference between adjacent frequency components is substantially constant and called the phase offset. In other words, individual frequency components are described by exp(i×ωjt+φj) where i is the index of the frequency component, ωj the angular frequency of the frequency component and φj the phase of the frequency component. In a looser terminology, an optical comb can also refer to a spectrum of multiple frequency components where the frequency difference between adjacent frequency components is substantially constant, but the phase difference between adjacent frequency components is not fixed to a constant phase difference. The former is referred to specifically as a mode-locked frequency comb because the phase difference between adjacent frequency components is locked to a fixed phase offset.
A semiconductor laser, such as an edge emitting Fabry-Perot laser, can lase simultaneously in multiple resonances of the laser cavity, thus forming an optical frequency comb in the loose sense. In particular, quantum dots provide a good gain material for such semiconductor comb lasers, as their gain spectrum is inhomogeneously broadened. This allows the laser to have stable emission in multiple cavity resonances without gain competition leading to unstable comb components. Semiconductor lasers can also be made to provide mode-locked frequency combs. One possibility is to incorporate a saturable absorber in the laser cavity. In other cases nonlinear processes inside the laser such as four wave mixing can lead to mode locking of the comb components. Such a comb laser is one type of frequency comb source.
An alternate way of generating optical frequency combs in a compact system is to utilize parametric generation in optical resonators, so called optical parametric oscillators (OPO). Of particular interest is the optical frequency comb generation inside chip-scale resonators integrated with other chip-scale optics, since this allows the realization of a compact, robust and cost-efficient system. One class of micro-resonators allowing chip scale frequency comb generation consists in whispering gallery mode resonators made out of silicon dioxide, silicon nitride or other chip scale dielectrics. The light from a laser with an emission spectrum corresponding substantially to a single frequency can be coupled to such an optical micro-resonator. The comb is generated via parametric light generation (e.g. four wave mixing). This results in a mode locked comb with the mode locking provided by the nature of the parametric generation processes. This is another class of frequency comb sources. The terminology micro-resonators is not restricted here to micron sized structures, but generally refers to chip scale resonators. The system can also be implemented with resonators that are not chip-scale.
Wavelength domain multiplexing (WDM) consists of transporting light with multiple frequency components in an optical fiber or an optical waveguide (a section of the optical link) where frequency components are independently modulated with separate data streams and thus form an independent data channel. Given an optical data modulation rate, the aggregate data transport rate of the system can be demultiplied by the number of independently modulated frequency components. The center frequency of the modulated frequency component is also referred to as the center frequency of the channel or the optical carrier frequency of the channel. Several channels, each corresponding to a different optical center frequency, can be transported through a given fiber, i.e. through a given optical link. Compact frequency comb sources such as comb lasers or micro-resonators are particularly useful in this context since they allow generating several frequency components in a single compact device.
Coherent detection is a technique by which the optical power efficiency of an optical link or of individual optical channels can be increased. That is to say, a lower amount of optical power is required for a channel with a given data rate. Coherent detection is based on optically mixing the incoming optical channel at the receiver with light generated by a local coherent light source, also referred to here as the local light source or the receiver light source. Mixing refers here to combining (summing) the incoming light with at least a portion of the locally generated light, thus generating an interference signal. This effectively reamplifies the optical signal, making it easier to overcome the electronic noise floor of the receiver.
The complex amplitude of the incoming optical channel is denoted as a×exp(iωat+iφa), where t is the time, a is a real positive number (that can depend on time, for example in case of amplitude modulation), φa is the phase of the incoming optical channel (that can depend on time for example in case of phase modulation) and ωa is the angular center frequency of the incoming optical channel. In the following, frequencies denoted by ω are taken by default to be angular frequencies. Similarly, the complex amplitude of the locally generated light with which the incoming optical channel is mixed is denoted as b×exp(iωbt+iφb). Mixing the incoming channel with the locally generated light and detecting the resulting power with a photodetector results in an AC electrical signal component proportional to a×b×cos(ωat−ωbt+φa−φb+θ), where θ is a phase dependent on the device used to implement the mixing, e.g. a Y-junction, a directional coupler or a multi-mode interferometer. θ can also be dependent on additional optical elements in the receiver, such as optical delays in the waveguide routing, for example in a 90 degree hybrid. If the local receiver light source is adjusted so that its emission frequency is equal to the center frequency of the incoming optical channel, one speaks of homodyne coherent detection. On the other hand, if the local receiver light source has a frequency that differs from the center frequency of the incoming optical channel, one speaks of heterodyne coherent detection. It can be seen that the incoming optical signal is essentially multiplied by b, the amplitude of the locally generated light. Additionally, the locally generated light can also provide a phase reference in case of phase shift keying (PSK), quadrature phase shift keying (QPSK) or other encoding schemes that use the optical phase as a degree of freedom to encode data.
A ninety degree hybrid refers to an optical device that allows to mix two optical signals a×exp(iωat+iφa) and b×exp(iωbt+iφb) and generate at least two signals with an instantaneous time varying optical power component proportional to a×b×cos(ωat−ωbt+φa−φb+θ) and an instantaneous time varying optical power component proportional to plus or minus a×b×sin(ωat−ωbt+φa−φb+θ), where θ is an implementation specific phase. Typically, a 90 degree hybrid creates four signals proportional to plus and minus a×b×cos(ωat−ωbt+φa−φb+θ) and to plus and minus a×b×sin(ωat−ωbt+φa−φb+θ).
A 90 degree hybrid can be realized by first splitting the incoming channel into two separate waveguides WG1 and WG2, by splitting the locally generated light into two different waveguides WG3 and WG4, by effectively adding a π/2 phase delay to one of the four optical paths (explicitly or by nature of the splitting/mixing devices), and by mixing WG1 with WG3 and WG2 with WG4. Another possibility is to implement a 90 degree hybrid with a 2 by 4 multi-mode interferometer.
In a homodyne coherent detector, ωat and ωbt cancel each other out. The electrical signal generated by the photodetector, a×b×cos(φa−φb+θ), is a baseband representation of the data in the sense that it is not multiplied with an intermediate frequency signal (IF) of finite frequency ωat−ωbt. For example, if the incoming channel is encoded with amplitude shift keying, φa−φb+θ can be chosen to be zero, so that the generated signal is a×b. The incoming signal has been amplified by b. If the incoming channel is encoded with a phase shift keying with φa, switching between φa−Δa/2 and φa+Δφa/2, φb−θ can be chosen to be equal to one of φa±π/2 so that the optical signal is converted into an amplitude coded electrical signal with maximized amplitude. For decoding a QPSK signal, a 90 degree hybrid is required. In this case the incoming optical channel and the locally generated light are interfered and detected so as to produce at least two separate AC signals in quadrature to each other, a×b×cos(φa−φb+θ) and one of a×b×cos(φa−φb+θ±π/2). This allows for example to detect a×b×cos(φa) and a×b×sin(φa) and to demodulate the QPSK signal. In a typical receiver the data is digitized by a thresholding circuit after demodulation. It is important to maximize the signal strength prior to the thresholding circuit in order to maximize the noise tolerance of the receiver.
If the optical mixing device is chosen as a 2 by 2 port device, two complementary optical signals are generated. Once detected by two separate photodiodes such as in a balanced receiver, two complementary AC electric signals are generated, a×b×cos(ωat−ωbt+φa−φb+θ) and −a×b×cos(ωat−ωbt+φa−φb+θ). By taking the difference between these two electrical signals, the signal amplitude can be doubled, thus increasing the optical power efficiency of the channel. In case of a 90 degree hybrid such as for QPSK demodulation, four AC signals can be generated, cos(ωat−ωbt+φa−φb+θ), −a×b×cos (ωat−ωbt+φa−φb+θ), a×b×cos (ωat−ωbt+φa−φb+θ+π/2) and −a×b×cos(ωat−ωbt+φa−φb+θ+π/2). By taking the difference between cos(ωat−ωbt+φa−φb+θ) and −a×b×cos(ωat−ωbt+φa−φb+θ) and by taking the difference between a×b×cos(ωat−ωbt+φa−φb+θ+π/2) and −a×b×cos(ωat−ωbt+φa−φb+θ+π/2) with two balanced receivers, the signal strength of the two AC components relevant to the QPSK signal can be doubled. In general, using balanced receiver architecture allows doubling the signal strength and cancelling DC offsets.
One difficulty with homodyne coherent detection is to phase and frequency lock the local light source with the center frequency and average phase of the incoming optical channel. This can be achieved with an optical phase locked loop (OPLL). In an optical phase locked loop, the light from the incoming optical channel is mixed with the locally generated light and converted into the electrical domain with a photodetector. The resulting signal contains an AC component whose frequency is proportional to the difference between the center frequency of the incoming optical channel and the frequency of the local light source. In the small signal limit, the time varying component is also proportional to the instantaneous phase difference between the incoming channel and the locally generated light (ωat−ωbt+φa−φb+θ). It serves as a feedback signal for a phase locked loop. The controlled oscillator of the phase locked loop is the local light source of the receiver, that can be tuned by a number of parameters. For example, in the case of a laser by changing its temperature, its injected drive current or by adjusting an additional laser control, such as the electrical signal applied to a phase tuning section if such a section is implemented. The feedback signal is typically low pass filtered before being applied to the local light source. An optical phase locked loop locks both the frequency and the phase of a single frequency local light source to the center frequency and average phase of an incoming optical channel with which the local light source is mixed as part of the OPLL. The resulting locally generated light is described by exp(iωat+iφa+θ) once the OPLL is locked. The phase difference θ is dependent on the implementation of the optics and can be chosen to warrant maximum data signal strength after coherent detection and demodulation.
OPLLs can be implemented with analog electronics, as well as with mixed signal or digital electronics. The latter requires digitizing the photodetected signal for further digital processing.
Due to the complexity of implementing an OPLL, it is sometimes easier or more desirable to use heterodyne coherent detection. In heterodyne coherent detection, the local light source at the receiver is left free running or is just coarsely adjusted to the frequency of the incoming optical channel. This results in the local light source emitting light with a different frequency than the center frequency of the incoming optical channel. This results in an AC signal a×b×cos(ωat−ωbt+φa−φb+θ) after photodetection where the term ωat−ωbt is non-zero. In order to demodulate the signal, the frequency difference ωa−ωb (called the intermediate frequency) has to be tracked as part of an electronic demodulation procedure. The center frequency of the incoming channel and the frequency of the local light source should be sufficiently close to each other for the electronics to be able to track ωa−ωb. This can be achieved for example by at least coarsely controlling or temperature stabilizing the local light source. The term ωat−ωbt results in an intermediate frequency modulation (IF) that is superimposed to the data modulation in the AC signal. An electronic phase locked loop (EPLL) is used to lock an electrical oscillator, e.g. a voltage controlled oscillator (VCO), to the IF signal. The signal from the electrical oscillator is referred to as the intermediate frequency reference or the recovered intermediate frequency carrier (these two terminologies are used interchangeably). It is then electrically mixed to the AC signal a×b×cos(ωat−ωbt+φa−φb+θ) to demodulate it into a×b×cos(φa−φb+θ+η), where η is a phase that is dependent on the implementation of the electric circuit and can be chosen to maximize the demodulated data signal strength. An EPLL can take the form of an analog PLL, a digital PLL or a mixed signal PLL.
Phase locked loops are sometimes operated as frequency locked loops in the loop start-up phase in order to increase their capture range. Once frequencies are locked or sufficiently close, operation is switched to phase locking so as to achieve both phase locking and frequency locking, since a frequency locked loop can only lock the frequencies together. This concept can be applied both to OPLLs and EPLLs.
A further method to demodulate a coherently detected signal is to use feed forward carrier recovery. In the terminology “feed forward carrier recovery,” carrier does not refer to the optical carrier of the incoming channel, but to the electrical carrier of the photodetected channel in the form of the intermediate frequency. This method has the same task than the OPLL or EPLL described previously in that it detects the phase and frequency of the intermediate frequency and uses this information to demodulate the signal. However, instead of relying on a feedback loop, feed forward data processing is employed. PLLs are very sensitive to delays in the feedback path making their realization challenging. Feed forward carrier recovery is a means to circumvent this difficulty. One way to implement a feed forward carrier recovery system is to digitize the signals generated by the photodetectors with high-speed analog to digital converters (ADC) and to use digital electronics, for example in the form of an FPGA, of a DSP processor or of a dedicated ASIC to implement the feed forward carrier recovery algorithm. The same architecture can also be used to implement a digital PLL after digitization of the photodetected signal by an analog to digital converter. The data can also be directly demodulated in the digital domain, in this case however one difficulty resides in the fact that the signals have to be typically sampled at the Nyquist rate and very high speed ADCs are required.
Coherent detection can be implemented in a WDM system. In a conventional WDM system the light for each communications channel is generated with a separate laser, since the light of different channels need to have distinct center frequencies. For each communication channel there also needs to be a corresponding locally generated light at the receiver with a close by frequency (heterodyne detection) or an identical frequency (homodyne detection). In a conventional WDM system, this would be implemented by providing a distinct local receiver laser for each optical channel. In addition, each of the channels need a distinct OPLL, EPLL or feed forward carrier recovery. This results in a high system complexity and a high number of costly components.
The novel system architecture disclosed here is particularly attractive if implemented with a high level of integration, since this allows achieving significant reduction of cost, size and electrical power consumption. One particularly attractive technology in this context is the realization of integrated optical components in silicon based technology, Silicon Photonics. It allows the realization of high-speed infrared optical detectors based on the integration of Germanium, of high-speed modulators for example in the form of Mach-Zehnder Interferometers (MZI) or resonant ring modulators, as well as filtering and frequency domain multiplexing/demultiplexing of light, for example with arrayed waveguide gratings, Echelle gratings or resonant ring based filters or add/drop multiplexing. By coupling several rings to each other rather than using a single ring, flat top filters and add/drop multiplexers can be realized allowing to relax the constraints on fabrication tolerances, laser frequency, temperature stabilization and control systems.
One of the drawbacks of Silicon Photonics is that it is a technology with high optical losses at fiber to chip interfaces and within optical modulators. For this reason, coherent detection, a method that allows increasing the optical power efficiency of optical links and thus to partially compensate for optical losses, is particularly attractive in this context. Since the demodulated signal is proportional to a×b as opposed to a2 as in an incoherently detected system, optical channels are only penalized by incurred losses with half the power penalty in dB.
Silicon photonics is also a technology that allows easily duplicating devices in a single chip, thus increasing system complexity at little additional cost and space. For this reason optical sources generating multiple optical frequency components such as comb lasers or optical parametric oscillators are particularly interesting when used together with Silicon Photonics. The frequency components of the optical comb source can be independently modulated inside a silicon photonics chip implemented at the transmitter and the multiple incoming optical channels can be independently coherently detected on a silicon photonics chip implemented at the receiver. Receiver and transmitter can be combined in a single Silicon Photonics chip at either end of a duplex link. Thus a complete WDM transceiver can be realized with a Silicon Photonics chip, an optical comb source and additional electronics. Silicon Photonics also allows single-chip integration of optics with electronics, thus allowing single-chip integration of the optics with a large portion of the required specialized electronics.